1. Field of the Invention
The present invention relates to a crystal and a method for the growth of a crystal. More particularly, it is concerned with a crystal article comprising a substrate on which single crystals and polycrystals are formed in a controlled manner, and a method for the formation of the same. 2. Related Background Art
Monocrystalline thin films used in semiconductor devices or optical devices have hitherto been formed by epitaxial growth on a monocrystalline substrate. For example, on a Si monocrystalline substrate (or silicon wafer), Si, Ge, GaAs or the like is known to be epitaxially grown from a liquid phase, gaseous phase or solid phase. On a GaAs monocrystalline substrate, monocrystals of GaAs, GaAlAs or the like are also known to be epitaxially grown. Using semiconductor thin films thus formed, semiconductor devices, integrated circuits, light-emitting devices such as semiconductor lasers and 1 LEDs, etc. are fabricated.
Recently, research and development were also made extensively on very high-speed transistors employing two-dimensional electron gas, super lattice devices utilizing a quantum well, but it is the high precision epitaxial technique as exemplified by MBE (molecular-beam epitaxy) using ultra-high vacuum, MOCVD (metal organic chemical vapor deposition), etc. that has made these possible.
In the epitaxial growth effected on such a monocrystalline substrate, it is required to adjust the lattice constant and thermal expansion coefficient between the monocrystalline material of the substrate and the epitaxial growth layer. An insufficiency in this adjustment may result in growth of a lattice defect in an epitaxial layer. It may also occur that the elements constituting the substrate are diffused in the epitaxial layer.
Thus, the conventional method of forming a monocrystalline thin film by the epitaxial growth is seen to greatly depend on the materials of the substrate. Mathews et al have examined the combination of the substrate material with the epitaxial growth layer (see EPITAXIAL GROWTH, Academic Press, New York, 1975, ed. by J. W. Mathews).
Also, size of the substate is 6 inches or so at present in the case of Si wafers, and more further progress is needed in making GaAs or sapphire substrates larger in size. In addition, the monocrystalline substrates involve high production cost, making high the cost per chip.
Thus, the formation of monocrystalline layers capable of fabricating devices of good quality according to the conventional methods has involved the problem that the kinds of substrates are limited to a very narrow range.
On the other hand, research and development have been also made extensively in recent years on three-dimensional integrated circuits formed by laminating semiconductor devices in the normal direction of a substrate to achieve a highly integrated and multifunctional state. Research and development are also extensively being made year by year on large area semiconductor apparatus in which devices are set in array on an inexpensive glass sheet, such as solar batteries and switching transistors for liquid crystal picture elements.
What is common to both of these is that needed are techniques by which a semiconductor thin film is formed on an amorphous insulating material and electronic devices such as transistors are formed thereon. Particularly sought after among these is a technique by which monocrystalline semiconductors of high quality are formed on an amorphous insulating material.
In general, the build-up of a thin film on the amorphous insulating material such as SiO.sub.2 may make amorphous or polycrystalline the crystalline structure of the built-up film for lack of long-distance order of the substrate material. Here, the amorphous film refers to a film kept in a state that the short-distance order as in most vicinal atoms is retained but there is no long-distance order more than that, and the polycrystalline film refers to a film in which monocrystal grains having no particular crystal direction have gathered in a manner separated at the grain boundaries.
For example, in an instance in which Si is formed on SiO.sub.2 by a CVD process, it forms amorphous silicon when the deposition temperature is about 600.degree. C. or less, and, when the temperature is more than that, it forms polycrystalline silicon having grain size distributed in the range of from several hundred to several thousand .ANG.. The Grain size and its distribution of the polycrystalline silicon may greatly vary depending on formation methods.
Polycrystalline thin films having a large grain size of the order of microns or millimeters have been obtained by fusing and solidifying amorphous or polycrystalline films by use of energy beams of lasers, rod-like heaters or the like (see Single Crystal Silicon on Non-single-crystal Insulators, Journal of Crystal Growth, Vol. 63, No. 3, October, 1983, edited by G. W. Cullen).
Measurement of electron mobility based on the properties obtained when transistors are formed on the thus formed thin films of each crystalline structure have revealed that there is attained a mobility of not more than 0.1 cm.sup.2 /V.sec in the case of amorphous silicon; a mobility of from 1 to 10 cm.sup.2 /V.sec in the case of polycrystalline silicon having a grain size of several hundred .ANG.; and, in the case of polycrystalline silicon having attained the large grain size by fusion and solidification, a mobility of the same degree as in the case of monocrystalline silicon.
It is seen from these results that there is a great difference in electrical properties between the device formed on the monocrystal area in crystal grains and the device formed across grain boundaries. In other words, the built-up films on amorphous substrates, which are obtained by conventional methods, have the amorphous structure or the polycrystalline structure with a grain size distribution, such that the devices formed thereon have much poorer performance as compared with the devices formed on the monocrystalline layers. For this reason, uses are limited to simple switching devices, solar cells, photoelectric transducers, etc.
Also, in the method of forming the polycrystalline thin films having a large grain size by fusion and solidification, the amorphous or monocrystalline thin film is scanned with energy beams for each wafer. Accordingly, there have been involved the problems such that the method requires much time to make the grain size larger, it is poor in mass-productivity, and it is not suited for achieving a large area.
As stated in above, the conventional crystal growth methods and the crystals fromed thereby can not readily achieve the three-dimensional integration and the large area and have difficulties in practical application to devices, so that it was impossible to easily form and with a low cost the crystals such as monocrystals and polycrystals which are necessary for the fabrication of devices having good properties and multi-functionality.